Lactic acid bacteria are found in many foods, for example, yoghurt, cheese or sauerkraut. These bacteria are desirable, as they change the taste and consistency of a raw material in such a way that a completely new foodstuff is created. In the manufacture of cheese, propionic acid bacteria also play an important role. They are responsible for the holes and the special taste of Emmental cheese. In a project of the Swiss National Science Foundation, researchers at the Laboratory of Food Biotechnology of ETH Zurich have discovered that these two groups of bacteria have other very useful properties.

Combination of bacteria is effective

In extensive studies, researchers at ETH have isolated over 1000 different bacteria from foodstuffs. They found 80 strains of lactic acid bacteria and ten strains of propionic acid bacteria that inhibited the growth of yeasts and moulds in yoghurt containing fresh fruit. A specific combination of lactic acid bacteria and propionic acid bacteria proved to be particularly effective. This culture extended the shelf-life of artificially contaminated yoghurt by more than two weeks.

Precise identification as the basis for safe bacterial strains

To be sure that the bacteria chosen are safe, harmless strains, researchers at ETH have accurately characterised and identified them. For this, they carried out microscopic investigations and biochemical tests. Additional detailed molecular biological investigations then enabled a very accurate and reliable identi-fication to be made.

Search for the active mechanism

Out of the originally large number of bacterial strains, a combination of two strains has recently successfully reached the market. It is now being sold as a protective culture. The work of ETH researchers on this is not yet at an end. In a follow-up project, they are working together with industry to investigate why this protective culture is so effective against yeasts and moulds. The first products of metabolism from the culture have already been isolated, but the precise mechanism and the interaction between the two strains is still an exciting mystery that ETH researchers want to solve.